1. Field of the Invention
The present invention is related to semiconductor materials, methods, and devices, and more particularly, to the growth and fabrication of semipolar (Ga,Al,In,B)N thin films, heterostructures, and devices.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [Ref x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The usefulness of gallium nitride (GaN) and alloys of (Ga,Al,In,B)N has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. As shown in FIG. 1, current state-of-the-art nitride thin films, heterostructures, and devices are grown along the [0001] axis 102 of the würtzite nitride crystal structure 100. The total polarization of such films consists of spontaneous and piezoelectric polarization contributions, both of which originate from the single polar [0001] axis 102 of the würtzite nitride crystal structure 100. When nitride heterostructures are grown pseudomorphically, polarization discontinuities are formed at surfaces and interfaces within the crystal. These discontinuities lead to the accumulation or depletion of carriers at surfaces and interfaces, which in turn produce electric fields. Since the alignment of these built-in electric fields coincides with the typical [0001] growth direction of nitride thin films and heterostructures, these fields have the effect of “tilting” the energy bands of nitride devices.
In c-plane würtzite (Ga,Al,In,B)N quantum wells, the “tilted” energy bands 104 and 106 spatially separate the hole wavefunction 108 and the electron wavefunction 110, as illustrated in FIG. 1. This spatial charge separation reduces the oscillator strength of radiative transitions and red-shifts the emission wavelength. These effects are manifestations of the quantum confined Stark effect (QCSE) and have been thoroughly analyzed for nitride quantum wells [Refs. 1-4]. Additionally, the large polarization-induced fields can be partially screened by dopants and injected carriers [Refs. 5, 6], so the emission characteristics can be difficult to engineer accurately.
Furthermore, it has been shown that pseudomorphic biaxial strain has little effect on reducing effective hole masses in c-plane würtzite (Ga,Al,In,B)N quantum wells [Ref 7]. This is in stark contrast to the case for typical III-V zinc-blende InP- and GaAs-based quantum wells, where anisotropic strain-induced splitting of the heavy hole and light hole bands leads to a significant reduction in the effective hole masses. A reduction in effective hole masses leads to a substantial increase in the quasi-Fermi level separation for any given carrier density in typical III-V zinc-blende InP- and GaAs-based quantum wells. As a direct consequence of this increase in quasi-Fermi level separation, much smaller carrier densities are needed to generate optical gain [Ref 8]. However, in the case of the würtzite nitride crystal structure, the hexagonal symmetry and small spin-orbit coupling of the nitrogen atoms in biaxially strained c-plane nitride quantum wells produces negligible splitting of the heavy hole and light hole bands [Ref 7]. Thus, the effective mass of holes remains much larger than the effective mass of electrons in biaxially strained c-plane nitride quantum wells, and very high carrier densities are needed to generate optical gain.
One approach to eliminating polarization effects and decreasing effective hole masses in (Ga,Al,In,B)N devices is to grow the devices on nonpolar planes of the crystal. These include the {11 20} planes, known collectively as a-planes, and the {1 100} planes, known collectively as m-planes. Such planes contain equal numbers of gallium and nitrogen atoms per plane and are charge-neutral. Subsequent non-polar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction. Moreover, it has been shown that strained nonpolar InGaN quantum wells have significantly smaller hole masses than strained c-plane InGaN quantum wells [Ref 9]. Nevertheless, despite advances made by researchers at the University of California and elsewhere [Refs. 10-15], growth and fabrication of non-polar (Ga,Al,In,B)N devices remains challenging and has not yet been widely adopted in the nitride industry.
Another approach to reducing polarization effects and effective hole masses in (Ga,Al,In,B)N devices is to grow the devices on semipolar planes of the crystal. The term “semipolar plane” can be used to refer to any plane that cannot be classified as c-plane, a-plane, or m-plane. In crystallographic terms, a semipolar plane would be any plane that has at least two nonzero h, i, or k Miller indices and a nonzero 1 Miller index.
Growth of semipolar (Ga,Al,In,B)N thin films and heterostructures has been demonstrated on the sidewalls of patterned c-plane oriented stripes [Ref 16]. Nishizuka et al. have grown {11 22} InGaN quantum wells by this technique. However, this method of producing semipolar nitride thin films and heterostructures is drastically different than that of the current disclosure; it is an artifact of epitaxial lateral overgrowth (ELO). The semipolar facet is not parallel to the substrate surface and the available surface area is too small to be processed into a semipolar device.
The present invention describes a method for the growth and fabrication of semipolar (Ga,Al,In,B)N thin films, heterostructures, and devices on suitable substrates or planar (Ga,Al,In,B)N templates in which a large area of the semipolar film is parallel to the substrate surface. In contrast to the micrometer-scale inclined-facet growth previously demonstrated for semipolar nitrides, this method should enable large-scale fabrication of semipolar (Ga,Al,In,B)N devices by standard lithographic methods.
Compared with zinc-blende InP- and GaAs-based quantum well heterostructures and devices, würtzite c-plane (Ga,Al,In,B)N quantum well heterostructures and devices require higher carrier densities to generate optical gain. This can be attributed to the presence of large polarization-induced electric fields and inherently large effective hole masses [Refs. 17, 18]. Therefore, reduction of built-in electric fields and effective hole masses is essential for the realization of high-performance (Ga,Al,In,B)N devices.
The design of typical InP- and GaAs-based heterostructure devices usually involves varying thin film parameters such as composition, thickness, and strain. By varying these parameters, it is possible to change the electronic and optical properties of individual epitaxial layers, such as bandgap, dielectric constant, and effective hole mass. Although not typically employed in InP- and GaAs-based device design, altering the crystal growth orientation can also affect the electronic and optical properties of individual epitaxial layers. In particular, altering the crystal growth orientation can reduce polarization effects and effective hole masses in nitride thin films and heterostructures. To accommodate this novel design parameter, we have invented a method for the growth and fabrication of semipolar (Ga,Al,In,B)N thin films, heterostructures, and devices. By properly selecting the correct substrate or semipolar template for crystal growth, the optimum combination of net polarization and effective hole mass can be chosen to suit a particular device application.
As an illustration of the effects of altering the crystal growth orientation, the piezoelectric polarization can be calculated and plotted as a function of the angle between a general growth direction and the c-axis for compressively strained InxGa1-xN quantum wells [Refs. 9, 18-20]. FIG. 2 shows the relationship between the conventional coordinate system (x, y, z) for c-plane crystal growth and a new coordinate system (x′, y′, z′) for a general crystal growth orientation. The conventional coordinate system (x, y, z) can be transformed into the new coordinate system (x′, y′, z′) by using a rotation matrix,
                    U        =                  (                                                                      cos                  ⁢                                                                          ⁢                  θ                  ⁢                                                                          ⁢                  cos                  ⁢                                                                          ⁢                  ϕ                                                                              cos                  ⁢                                                                          ⁢                  θ                  ⁢                                                                          ⁢                  sin                  ⁢                                                                          ⁢                  ϕ                                                                                                  -                    sin                                    ⁢                                                                          ⁢                  θ                                                                                                                          -                    sin                                    ⁢                                                                          ⁢                  ϕ                                                                              cos                  ⁢                                                                          ⁢                  ϕ                                                            0                                                                                      sin                  ⁢                                                                          ⁢                  θ                  ⁢                                                                          ⁢                  cos                  ⁢                                                                          ⁢                  ϕ                                                                              sin                  ⁢                                                                          ⁢                  θ                  ⁢                                                                          ⁢                  sin                  ⁢                                                                          ⁢                  ϕ                                                                              cos                  ⁢                                                                          ⁢                  θ                                                              )                                    (        1        )            
where φ and θ represent the azimuthal and polar angles of the new coordinate system relative to the [0001] axis, respectively. As shown in FIG. 2, the z-axis corresponds to the [0001] axis 102 and the z′-axis 200 corresponds to the new general crystal growth axis. For calculating physical parameters, dependence on the azimuthal angle (φ) 202 can be neglected because the piezoelectric effect in würtzite materials shows monoaxial isotropic behavior along the [0001] axis [Ref 21]. Thus, a family of equivalent semipolar planes can be uniquely represented by a single polar angle (θ) 204, referred to hereafter as simply the crystal angle 204. The crystal angles 204 for polar, non-polar, and a few selected semipolar planes are shown in Table 1 below.
TABLE 1List of polar, non-polar, and selected semipolarplanes with corresponding crystal angles.PlaneCrystal Angle (θ){0001}  0°{10 14}25.1°{10 13}32.0°{10 12}43.2°{20 23}51.4°{11 22}58.4°{10 11}62.0°{20 21}75.0°{1 100} 90°{11 20} 90°
As expected, the {0001} planes correspond to θ=0°, the {1 100} and {11 20} planes correspond to θ=90°, and the semipolar planes correspond to 0°<θ<90°.
The piezoelectric polarization of a crystal is determined by the strain state of the crystal. For heteroepitaxial growth of non-lattice matched crystal layers, the strain state of the individual layers is determined by the biaxial stress in the growth plane.
For a general crystal growth orientation along the z′-axis 200, the biaxially stress components σx′x′ and σy′y′ in the growth plane can be transformed into the conventional (x, y, z) coordinate system through the transformation matrix U. This allows the determination of the strain state and piezoelectric polarization in (x, y, z) coordinates. Thus, the piezoelectric polarization in (x, y, z) coordinates varies as function of the crystal angle (θ) 204 through the transformation matrix U. For a general crystal growth orientation, the piezoelectric polarization can be obtained by taking the scalar product between the polarization vector P in (x, y, z) coordinates and the unit vector {circumflex over (z)}′ along the general crystal growth direction:Pz′=P·{circumflex over (z)}′=Px sin θ+Pz cos θ  (2)
where Px and Pz represent the components of the piezoelectric polarization in (x, y, z) coordinates and are in general dependent on the crystal angle (θ) 204, as described above.
FIG. 3 illustrates the piezoelectric polarization 300 as a function of the angle between the growth direction and the c-axis for compressively strained InxGa1-xN quantum wells with unstrained GaN barriers [Refs. 9, 18-20]. As expected, the polarization 300 is maximum for c-plane growth (θ=0°) and zero for a-plane or m-plane growth (θ=90°). In between these two limits, the polarization changes sign once and is equal to zero at some angle θo 302. The exact value of θo 302 is dependent on the values of several physical parameters such as the piezoelectric tensors and elastic constants, many of which are largely unknown at present [Refs. 21-25].
Much like piezoelectric polarization effects, effective hole masses for compressively strained InxGa1-xN quantum wells can also be substantially reduced by altering the crystal growth orientation. Theoretical results [Ref. 9] show that the effective hole masses for compressively strained InxGa1-xN quantum wells should decrease monotonically as the crystal angle is increased due to anisotropic strain-induced splitting of the heavy hole and light hole bands. Thus, growing compressively strained InxGa1-xN quantum wells on semipolar orientations should significantly reduce effective hole masses, especially on orientations with large crystal angles.